Furnace construction



June 3, 1958 .1.w. THRocKMoRToN ETAL 2,837,065

FURNAE CONSTRUCTION 5 Sheets-Sheet 1 Filed Jan. 8, 1953 .57 /Vd/lllsJune 3, 1958 J. w. THRocKMoR-roN ET Al. 2,837,065

u FURNACE CONSTRUCTION 1 Filed Jan. 8, 1955 3 Sheets-Sheet 2 IN VEN TORSfrn/(makin BY J0 J1 /dllzs June 3, 1958 J. W. THROCKMORTON ET A1. 2,837,065

FURNACE CONSTRUCTION Filed Jan. 8, 1953 5 Sheets-Sheet 3 United i StatesPatent FURNCE CONSTRUCTION John w. Thmclmwrto'nv and John s. wams, NewYork,

NfY., assignors to Petro-Chem Process Company, Incorporated, New York,N. Y., a corporation of Delaware Application January s, 195s, serial No.330,192l

4 claims. (cl. 12g- 356) Our invention pertains to verticalfheaters andfurnaces having a cylindrical bank of vertical tubes anda central ame bywhich the tubes are heated largely by radiation.

InV heaters of this type such as thosev shown in our U. S. Patents2,333,077, issued October 26, 1943, and 2,340,287, issued February 1,y1944, the tubes are arranged in a single cylindrical bank and aresurrounded by' .a refractory lining Within a metal cylinder which formsthe furnace wall.

Burners are located at the bottom or at the top and produce a centralame which imparts its intense heat very efciently to the tubes and tothe exposed portion of the refractory furnace lining. by radiation.

The tubes have been uniformly spaced and we have discovered thatthespacing of the tubes makes a great difference as to the quantity ofheat-absorbed directly through the exposed: surfaces of the tubes and asto the quantity of heat reflectedA fromr the hot refractory wallsto theback surfaces of the tubes.

If the uid to be heated such as; hydrocarbon oil or Water enters thetube bank at a low temperature, say 300 F. and leaves at a hightemperature, say l000 F. to l500 F., the tubes having; the lowesttemperature ui'd will absorb the greatest heat quantity and the tubeshaving the highest temperature huid will absorb the least quantity ofheat.

The object of our invention is to provide such a spacing of the tubes asto obtain a predetermined optimum input to each tube of the heater or toeach group of tubes although the conditions of temperature and the phaseof the uid being heated and the rate lof absorption vary widely.

Some of the advantages of our invention may be obtained by an equalspacing of thetubes in each of several groups of tubes and varying thespacing in each group as compared to the other groups. For example, thetube bank may be divided into three sectors, the tubes in the inletsector being spaced close together, for example, 41/2 O. D. tubes on 8"centers and the middle sector on 12" centers and the high temperatureoutlet sector on `16" centers.

A greater degree of refinement with increased advantages may be securedby plotting a curve of heat input desired, as hereinafter explained, andvarying the tube spacing from the inlet end of the tube bank to theoutlet end according to the conditions which obtain in each tube.

It is usually preferable from the standpoint of mc- Vchanical simplicityto divide the tube bank into several sectors with the tubes in eachsector uniformly spaced thus making it possible to use uniform returnbends in Veach sector and the number of sectors will depend on the sizeof the furnace and the requirements of the operation being conducted inthe heater.

If for example an oil cracking operation is carried on, the temperatureand phasewhether liquid, vapor -or lmixedthe outlet temperature andthetype of oil to be treated will Vary the optimum conditions. Some heatersare designed to discharge the oil from the co1l immediately uponreaching top temperature while others are designed to level out and holdthe temperature constant towards the outlet in order to provide apredetermined soaking time.

The tube temperature on the tube face directly exposed to the radiantheat of the ame is important to control 'so as toV avoid overheating andif an optimum tube surface temperature is determined it is thendesirable to insure that the indirect heat imparted to the back surfacesof the tubes by reradiation from the furnace wall is increased to getthe maximum overall input.

Thus a further object is to provide a tube distribution which will givepredetermined heat transfer rates which may vary from one tube toanother.

Referring to the drawings:

Fig. l is a sectional elevation of a heater embodying our invention;

Fig. 2 is a cross section on the line 2 2 of Fig. l;

Fig. 3 is a cross section on the line 3 3 of Fig. l;

Fig. 4 is a graph showing an example of applying the invention to theheating of gas in a cracking process; and

Fig. 5 is a cross section of a tube circle corresponding to that of Fig.3 in which the tube spacing varies grad-v ually in a predetermined wayas per curve B in Fig. 4.

The heater shown comprises a vertical cylindrical shell ll supportedabove the foundation 2 vby legs 3 and provided with an inner lining 4 ofrefractory material which is preferably spaced from the steel shell byinsulation 7. A cylindrical upper furnace section 5 is coaxial with themain furnace shell l and is supported thereby.

Within the main furnace shell is a cylindrical bank of vertical tubes 6,which are spaced from one another as shownv in Fig. 3 and located near,but spaced inwardly from, the refractory lining. A large open combustionchamber 8 is thus formed.

The furnace has a bottom plate 9 in which a ring of burners 10 is set soas to provide an upshot flame centrally within the chamber.

An inner concentric cyclindrical refractory Wall 11 is mounted withinthe upper furnace section 5 and together with a cylindrical baie 29,forms an annular convection space in which fin tubes 12 are mounted andmay constitute a preheater for the fluid supplied to the main furnacetubes.

A conical bale 13 is supported by a ring 30 and hangs down into thefurnace at the top and forces the hot gases to flow through openings 31in the ring 30 and into and through the annular space around the iintubes 12. The arrangement of the preheater tubes is shown in Fig. 2.

The upper end of the convection annulus is connected with the stack 14.

Referring particularly to Fig. 3, the oil or other uid to be heated isintroduced at inlet tube 15 by pipe 16 (Fig` l) from the preheater andthen ows in series through successive tubes 1'7 to the outlet tube 18 ofthis low temperature sector of the tube bank. Thence the oil enters tube19 and fiows successively through tubes 2i) to outlet tube 21 of thisintermediate sector of the tube bank. The oil then enters tube 22 andows successively through tubes 28 of the high temperature sector of thebank to outlet tube 23. The tubes are connected at their ends in theusual manner by return bends indicated at 24 in Fig. l.

The graph of Fig. 4 shows the lluid temperature in degrees Fahrenheitagainst the percentage heating surface of the tube bank underconsideration. A hydrocarbon gas is the iiuid in the tube bank beingheated to high temperatures during a gas cracking operation.

Curve A shows conditions when four diameter spacing Patented June 3,1958:

is used throughout the tube bank, curve C shows conditions when twodiameter spacing is used throughout, and curve B shows conditions whenvariable spacing is employed in accordance with this invention. Y

The limitations of two diameter spacing (curve C) are apparent in 'thatthe maximuml temperature for a givenamount of heat input, is limitedwell below that where four diameter spacing is used (curve A)- Among theyobjections to using four diameter spacing is that a great increase inthe size of the furnace lfor a given capacity tube bank, is necessarydue to the large increase in space consumed by the same numberof tubesas when two diameter spacingl is used.

Therefore by using a graduated spacing of from two to four diameters(curve B), substantially .the same tem-` perature may be reached as withfour diameter spacing only; and, at the same time, the space necessary(size of the furnace) is not greatly increased over that for a furnaceusing two diameter spacing This is one of the advantages to be gained byusing variable spacing in accordance with this invention. t

As will be discussed below, the reason that the higher temperatures maybe reached with the wider spacing is that the unit of tube surface ismuch more eective at the wider spacing 4and is especially desirable atthe higher Huid temperatures where the diiference between the flametemperature and the uid temperature is much less.

The purpose of using wider than normal spacing is to provide means forimproving radiant heat absorption by the whole circumference of thetubes obtained by increasing the eiectiveness of the back or theshielded portion of the tube circumference, which normally does notreceive any appreciable direct radiation from the source, and

gets only a small amo-unt of the total available as reradiation fromrefractories behind the tubes.

YIn normal practice the tubes in the radiant section of tubular heatersare located with a center to center spacing of 1.75 to 2.25 diameters.Under these conditions, using a spacing of 2 as an example, thev-relative heat input intensity or ux distribution varies from a maximumof unity at the front, all due to direct radiation from' the source infront of the tubes, to 0.34 at the back, all due to reradiation from therefractories behind the tubes. Intermediate points receive directradiation and reradiation in varying proportions. The average intensityalong the whole circumference for this spacing is 0.56 with the front ofthe circumference, the exposed half, receiving 70 percent of the total,or approximately 2.3 times as much as the -shielded half. averageintensity is 1.79.

The eiectiveness of the shielded half of the circumference and thereforeof the whole circumference can be increased by increasing the spacingbetween the tubes. If the spacing is increased to 4 diameters and withthe same maximum intensity of unity at the front of the tube theintensity at the back is 0.63, or approximately twice as great as thatwith a spacing of 2. The average intensity is 0.77, of which the exposedhalf receives 58 percent, or only 1.37 times as much as the shieldedhalf, and the ratio of maximum to average intensity is 1.3. With aspacing of 4 and the same maximum intensity at the front, the whole tubecircumference is 37 percent more effective than with a spacing of 2.

Table I shows two examples of the eiects of spacing on radiant heatabsorption rates. With values in column A as a basis, column B-l showsthat with the same intensity of tiring, i. e. 30,000 B. t. u./hr./sq.ft. of effective surface, and the same maximum intensity of 18,500 B. t.u./hr./sq. ft. at the front, the tubes on a spacing of 4 will haveaverage absorption rates of 14,000 B. t. u./ hr./sq. ft. ofcircumferential surface as against 10,300 with a spacing of 2. If theaverage rates on the exposed half of the circumferenceand not themaximum intensity at the front are the determining factor, then, asshown in The ratio of maximum to column B2, for the same average of7,200 B. t. u./hr./sq. ft. on the exposed half the tubes with a spacingof 4 will have average rates for the whole circumference of 12,400, or20 percent higherthan with spacing of 2 diameters (see column A) andwill obtain these higher rates with milder ring (25,000 vs. 30,000),lower residual gas temperatures (1,400 Vvs. 1,460), and lower maximumintensity (16,100 vsl 18,400). The relationship as shown by comparisonof column A with column B-2 is of particular importance in someIliighrtemperature pyrolysis where higher rates towards the end ofthecoil are required but where with the normal -spacing this may result inhigher maximum intensities than the alloy can safely withstand.

The values in Table I apply when the radiant heat absorption rates areuniform through the Whole bank and where all tubes inthe bank are at thesame temperature. In mostv applications metal temperatures increase frominlet-to outlet;k Also, in a large number of applications it isdesirable to' apply increasingly higher heat input rates towards the endof the coil to compensate for higher heat requirements per degreetemperature rise of the fluid bein heated as it travels to the end ofthe coil.

Table 1I shows the effect vof increased spacing with increasing metaltemperatures. The table shows that with the'same intensity of firing,25,000 B. t. u./hr./sq. ft. of effective area, by increasing the spacingfromV 2D to 3D the total radiant heat transfer rates can be increasedapproximately 23 percent even though the metal temperature of the tubeswith a-wider spacing is 1000 F. If the spacing is increased to 4D, therates could be increased 37 percent over a spacing of 2D even though thetemperature of the metal of the tubes with 4D spacing is increased from800 to l200 F.

The foregoing analysis demonstrates that a change of spacing through thecoil, gradual or in steps, can be used to meet the needs 'of any processwith increasing or decreasing heat requirements towards the end of thecoil, or, as shown on Table Il, vto reduce the maximum intensity of heatapplied to the tubes while maintaining the same or higher average ratesof heat input. The Vlatter is particularly valuable in high-temperaturepyrolysis, such as production of ethylene and other Petro-chemicals, orlin superheating of heat-carrying iluidsto high temperatures where themaximum permissible heat intensity is the limiting factor of the overallrates which can be used without exceeding the maximum allowabletemperature for the alloy.

It will be evident from the above that this invention may be adaptedV tovarious uses. For example, inrsome instances it might be desirable tohave wide spacing at the input end of a coil or bank of tubes, and thento decrease the spacing toward the output end ofthe bank.

While we have described the best mode `of using our vinvention as we nowsee it, this should not be taken as limiting our invention in any waybut merely as a description thereof.

TABLE I i A B-l B-2 1. Liberation, B. t. u./hr./sq. ft. of effectivearea of bank... 30,000 30, 000 25, 000 2. Flame burst temperature, F-..3, 400 3, 400 3, 400 3. Metal temperature, F 800 800 800 4. Absorption,B. t. u./hr./sq. ft. oi effective area of bank 18, 500 18, 500 16, 2005. Equilibrium gas (residual) temperature of gases, 1, 460 1, 460 A1,400Absorption, B. t. u./hr./sq. it. ofeircumferential area 10,300 14,10012, 400 7. Maximum intensity (B. t. u./hr./sq. ft.) 18, 400 18, 400 16,8. Absorption by exposed bali B. t. u./h.r./sq. Y

ft 7, 200 8, 200 7, 200 9. Absorption by shielded halt. 3, 100 5,900 5,200

*(D=outside diameter of tubes.)

We claim:

1. A vertical tube iiuid heater comprising an upright cylindricalfurnace shell having a ceramic lining, a single cylindrical bank ofvertical tubes within the furnace shell, said tubes being close to butspaced from the ceramic lining, and means for connecting the tubes inseries to form a hollow cylindrical heating coil, upshot burners at thebottom of the furnace adapted to discharge llame and hot gases axiallyupward within the heating coil, whereby the tubes are heated by directradiation from the ilame and hot gases and by reradiation from theceramic lining, the tubes of said hollow cylindrical coil spacedrelatively close together near the inlet end of the coil where the uidto be heated enters at a low temperature, and relatively far apart nearthe outlet end of the coil where the uid has already reached acomparatively high temperature.

2. A vertical tube uid heater comprising an upright cylindrical furnaceshell having a ceramic lining, a single cylindrical bank of verticaltubes within the furnace shell, said tubes being close to but spacedfrom the ceramic lining, means for connecting the tubes in series toform a hollow cylindrical heating coil, upshot burners at the bottom ofthe furnace adapted to discharge tlame and hot gases axially upwardWithin the heating coil whereby the tubes are heated by direct radiationfrom the llame and hot gases and by reradiation from the ceramic lining,the tubes of said hollow cylindrical coil spaced relatively closetogether near the inlet end of the coil where the uid to be heatedenters at a low temperature and relatively far apart near the outlet endof the coil where the uid has already reached a comparatively hightemperature, and intermediate tubes spaced apart more widely than thosenear the inlet and less Widely than those near the outlet end of thecoil.

3. A vertical tube uid h cater comprising an upright cylindrical furnaceshell having a ceramic lining, a single cylindrical bank of verticaltubes within the furnace shell, said tubes being close to but spacedfrom the ceramic lining, and means for connecting the tubes in series toform a hollow cylindrical heating coil, upshot burners at the bottom ofthe furnace adapted to discharge llame and hot gases axially upwardwithin the heating coil, whereby the tubes are heated by directradiation from the ame and hot gases and by reradiation from the ceramiclining, the tubes of said hollow cylindrical coil spaced atprogressively greater width *from the inlet end to the outlet end oi thecoil so that the percentage of heat applied to the tubes by reradiationrelatively to the heat applied by direct radiation is increased as thetemperature of the iluid being heated is increased.

4. A vertical tube lluid heater comprising an upright cylindricalfurnace shell having a ceramic lining, a single cylindrical bank ofvertical tubes within the furnace shell, said tubes being close to butspaced from the ceramic lining, means for connecting the tubes in seriesto form a hollow cylindrical heating coil, upshot burners at the bottomof the furnace adapted to discharge ilame and hot gases axially upwardwithin the heating coil, the tubes of said hollow cylindrical coilspaced relatively close together near the inlet end where tluid to beheated enters at a low temperature and relatively far apart at theoutlet end, and intermediate tubes spaced apart more widely than thosenear the inlet and less widely than those near the outlet whereby thetubes are heated by direct radiation from the flame and hot gases and byreradiation from the ceramic lining, the percentage of heat applied byreradiation to that applied by direct radiation being increased as thespacing between the tubes of the coil increases.

References Cited in the lile of this patent UNITED STATES PATENTS1,804,155 DeFlorez May 5, 1931 1,828,814 Lucke Oct. 27, 1931 2,112,224Alther Mar. 29, 1938 2,333,077 Wallis et al. Oct. 26, 1943 FOREIGNPATENTS 463,549 Italy May 12, 1951

